As is known, helicopters are normally equipped with a number of transmissions for transmitting power from one or more turbines to the main and/or tail rotor, and/or to various accessory devices, e.g. for powering operation of on-board instruments.
Lubricating fluid, typically oil, is circulated in known manner inside the transmission to both lubricate and cool the moving parts.
For effective lubrication and cooling, the lubricating fluid circulating inside the transmissions must be cooled.
So, helicopters are equipped with cooling systems substantially comprising:                a heat exchanger for exchanging heat between the transmission oil and the air circulating inside the cooling system; and        a fan for creating airflow from the heat exchanger to the fan itself.        
More specifically, the airflow draws heat from the heat exchanger, and hence the transmission, and flows over the fan at a temperature of about 125° C.
The cooling systems also comprise:                a casing;        a shaft connected to a drive member to rotate the fan; and        one or more bearings supporting the shaft with respect to the casing.        
The hot airflow from the heat exchanger to the fan heats the area around the bearings, thus reducing their working life and dependability.
A need is therefore felt within the industry for a system of cooling helicopter transmissions without impairing the working life and dependability of the fan shaft bearings.
Known helicopter transmission cooling systems are described in GB 591,982 and KR-A-20100109717.
EP-A-2409919 discloses a system for cooling a transmission of an aircraft, comprising a stator, a fan for creating a current of a heat-carrying fluid towards the heat-exchanger, a shaft rotating about an axis to rotate an impeller of the fan, and a bearing supporting the shaft in rotation about the axis and with respect to the stator.
Due to the fact that the fan creates a current of the heat-carrying fluid directed towards the heat-exchanger, the bearing of EP-A-2409919 does not run a substantial risk of overheating.